1. Field of the Invention
The present invention relates in general to a method for coating thermoplastic resin beads for use in imitation pearls. More specifically, surface of celluloid is treated with a surface treatment compound (A) having been dissolved into a mixed solution of acetone, ethylacetate, butylacetate and benzene, and the surface treated celluloid is coated with: a compound for an under coat (B), a compound for a mid coat (C), and a compound for a top coat (D), wherein each of the compounds (B), (C), and (D) is prepared by adding a pigment to a mixed solution of ethylacetate and amylacetate.
2. Description of the Related Art
In general, thermoplastic resins are very sensitive to chemicals, and thus it was difficult to coat beads with those resins. Even though the coating process was succeeded, it did not increase merchantability of the beads.
However, industrial development has succeeded to mass-produce nontoxic and environmentally friendly imitation pearls made of thermoplastic resin. Moreover, the coating method of the mass-produced pearls was very easy and gave excellent color and luster to the pearls. Thusly produced pearls were high quality and hardly defective.
Industrial development also brought positive changes on ornaments and raised more interests in accessories. As people live in more comfortable circumstances than the past, their interests in ornaments and accessories have grown naturally.
Among them is the imitation pearl. As described above, imitation pearls could be mass produced and are no way inferior to natural pearls in terms of the texture, so they are widely used for accessories, necklaces, rings, brooches, toys and so on.
To make imitation pearls for ornamental purposes, cellulose nitrate (or simply, cellulose) is dissolved in diverse solvents, and pigments are added thereto. Then, resin beads are coated with the mixture of the cellulose solution and the pigments.
In the past, basic lead carbonate (2PbCO3Pb(OH)2), which is a compound of a lead (Pb)-containing neutral salt and a hydroxide, was usually used for the pigment to be added to the cellulose solution. Although the basic lead carbonate was effective for creating almost same texture and luster with natural pearls, it had deadly effects on a human body and was a major environmental contaminant.
To resolve the above-described problems, a new technique has been recently proposed. According to the new technique, cellulose was dissolved in a solvent of acetone or butylacetate, and a harmless pigment, bismuthoxychloride (BiOCl), was added to the cellulose solution.
However, as long as nitrocellulose itself was used, solubility and dispersibility of pigments based on titanium dioxide and bismuthoxychloride were poor. Also, imitation pearls made of nitrocellulose were easily turned to yellow because of nitrocellulose being very sensitive to ultraviolet rays. Although there was another technique introduced to get the same texture and physical properties with natural pearls by mixing titanium dioxide with a specific pigment (highlight pearl green), its manufacturing process was very complicated, and matching appropriate components and contents therefor was often failed. As a result, it seemed to be almost impossible to manufacture imitation pearls having gentle and bright colors like natural pearls, and this inferiority only deteriorated merchantability of the imitation pearls.
In the past, high impact non-coated thermoplastic resins like acryl, ABS, and PS were used as the materials of the bead. However, when the surface of the bead was polished, the bead still showed weakness to chemicals during the coating process. Needless to say, the bead was low in quality.